1. Field of the Invention
The present invention relates to a sintered product of silicon nitride that can be favorably used as parts for heat engines, such as parts of engines and parts of gas turbines. More specifically, the invention relates to a sintered product of silicon nitride having a high strength over a wide temperature range of from normal temperature to high temperatures, and exhibiting excellent static fatigue property and oxidation resistance.
2. Description of the Prior Art
The sintered product of silicon nitride has a high strength and has heretofore been drawing attention as a material that exhibits excellent heat resistance, resistance against thermal shock and oxidation resistance. Therefore, study has been forwarded to use the sintered product of silicon nitride as parts for heat engines, such as engineering ceramics and, particularly, as parts for gas turbines and engines, as well as for automotive parts.
The silicon nitride itself is a material that can be difficultly sintered. Accordingly, a highly dense and highly strong sintered product of silicon nitride has been obtained by mixing a sintering additive such as an oxide of a rare earth element, aluminum oxide or magnesium oxide into the silicon nitride, and firing the mixture. For example, a mixture powder is prepared by adding the sintering additive to the powder of silicon nitride, and is molded into a predetermined shape, followed by firing in a non-oxidizing atmosphere such as of nitrogen at a temperature of from 1600 to 2000.degree. C. to prepare a sintered product of silicon nitride.
In a nitrogen atmosphere under normal pressure, the silicon nitride undergoes the decomposition at a temperature of higher than 1800.degree. C. Usually, therefore, the silicon nitride is fired in a pressurized nitrogen atmosphere while suppressing the decomposition of the silicon nitride, thereby to realize the firing at a high temperature and to obtain a sintered product of silicon nitride featuring excellent strength at high temperatures.
Further, it has been known to obtain a sintered product of silicon nitride having excellent strength at high temperatures by crystallizing the sintering additive present on the grain boundaries of the silicon nitride crystal phase, so that: crystal phases such as melillite (RE.sub.2 O.sub.3.Si.sub.3 N.sub.4) and wollastonite (RESi.sub.2 N) are precipitated on the grain boundaries and that the heat resistance is heightened on the grain boundaries.
However, the above-mentioned conventional sintered product of silicon nitride has problems as described below.
By using, for example, an oxide of a rare earth element, aluminum oxide or magnesium oxide as a sintering additive, it is allowed to prepare a highly dense sintered product which exhibits increased strength at normal temperature. The sintering additive has a low melting point enabling the firing to be conducted at low temperatures. It is therefore allowed to suppress the growth of silicon nitride particles during the firing and, hence, to further increase the strength of the sintered product at normal temperature. However, since the sintering additive has a low melting point, the grain boundary phase in the sintered product is softened even at low temperatures. Accordingly, the sintered product may exhibit increased strength at normal temperature but exhibits decreased strength at high temperatures.
Further, when the sintered product of silicon nitride is to be prepared by firing at a high temperature based on the pressurized sintering, the sintered product may exhibit increased strength at high temperatures making, however, it difficult to control the growth of silicon nitride particles, and increased strength cannot be expected at normal temperature.
In a sintered product of silicon nitride precipitating the crystal phases such as melillite and wollastonite on the grain boundaries, further, the grain boundary phase is softened in a suppressed manner at high temperatures and, hence, an increased strength is exhibited at high temperatures. This sintered product can be produced while controlling the growth of silicon nitride particles, and a large strength is exhibited at normal temperature. This sintered product does not undergo creep deformation or creep destruction since the grain boundary phase is suppressed from being softened, but develops a static fatigue due to sub-critical crack growth (hereinafter referred to as SCG) without accompanied by deformation, arousing a problem in that the time to failure is shortened at high temperatures. Besides, since the crystal phases precipitated on the grain boundaries have poor resistance against the oxidation, the grain boundary phase is preferentially oxidized, deteriorating corrosion resistance in a high-temperature oxidizing atmosphere.